1. Field of the Invention
The present invention relates to an apparatus for processing foam on an alkaline solution that has been used for cleaning an object to be cleaned, such as a metal strip in the iron-manufacturing industry.
2. Description of the Related Art
In general, an alkaline solution for cleaning an object, such as a metal strip in the iron-manufacturing industry, is used in a desired cleaning process and the solution is then recovered in a storage tank. Subsequently, the alkaline solution is returned and reused in the object cleaning process. The alkaline solution is generally recycled a plurality of times.
On the alkaline solution recovered from the object cleaning process in the storage tank, a large amount of floating foam is present. The foam includes impurities such as oil. As is well known to those skilled in the art, these impurities significantly impair the cleaning performance of the object to be cleaned using the alkaline solution, and therefore, must be removed.
Hitherto, in order to remove the foam from an alkaline solution that has been used for cleaning a metal strip in the iron-manufacturing industry, a foam phase floating on the liquid surface level of the alkaline solution in a storage tank is separated from the liquid. The methods are broadly divided into the following types (1) and (2):
(1) A method including the step of removing the foam on the alkaline solution in the storage tank with a suction unit.
(2) A method including the steps of sucking the foam on the alkaline solution in the storage tank, defoaming the foam by centrifugal separation, and returning the total mixture of the fine foam formed by the defoaming step and the liquid to the storage tank.
In method (1), the foam overflows from the storage tank. Unfortunately, the overflowed foam contaminates the surrounding floor face, and in addition, a drainage ditch of the solution is covered with the overflowed foam. In this method, the amount of waste of the foam is significant and is as large as about 10 to about 20 tons per day. Accordingly, this method causes a serious pollution problem due to the overflowing of the foam.
In method (2), the foam generated in the storage tank and the foam generated from the mixture solution that is returned to the storage tank are mixed, thereby increasing the amount of foam. Furthermore, as the amount of foam increases, the foam quality is deteriorated. When the above-described steps of sucking the foam, defoaming the foam, and returning the solution (i.e., returning the mixture to the storage tank) are repeated for about 10 days, the foam is changed to a soft cream-like scum. (In the iron-manufacturing industry, the operation is actually continued for at least 15 to 30 days.) The deterioration of the foam quality contaminates the alkaline solution in the storage tank. This phenomenon is not preferable to the cleaning of an object to be cleaned.
In order to solve this problem in method (2), when the mixture is not returned to the storage tank but disposed of, the amount of waste is significant and is as large as about 10 to about 20 tons per day, thus causing many problems. Furthermore, in order to compensate for the loss in detergency, a detergent is added to the alkaline solution that is used repeatedly. In the above case, the consumption of the detergent is significantly increased. Also, the cost is significantly increased.
In other words, an effective processing of foam on the alkaline solution in the storage tank, the decrease in the detergent consumption, and an effective and highly practical removal of the scum have not been achieved yet by the known methods.